In order to optimize extraction of hydrocarbons from an underground formation, it is known how to take material samples from the underground formation (core samples), or else use so-called “analog” porous materials, either consolidated or not, and saturate them with water and hydrocarbons thereby simulating the composition of a petroleum reservoir. Various extraction means may then be tested on these samples: water, steam, surfactants, polymers, gases . . . . During each experiment, fractions are retrieved, essentially comprising hydrocarbons mixed with water and mineral solids. By accurately assaying the amount of hydrocarbons present in each fraction, it is possible to obtain a simulation of the production history and thereby assess the efficiency of the tested extraction means.
An accurate assay of the amount of hydrocarbons in a mixture with water and possibly mineral solids is also useful during the production from the deposits. When dealing with not very viscous conventional crude oil, simple decantation or centrifugation may give the possibility of separating these hydrocarbons from the water and of thus carrying out the required assay. However, it happens that, even with not very viscous oils, persistent emulsions form and prevent accurate assaying of the produced hydrocarbon. When one is in the presence of heavy oils, the situation is even more difficult: indeed large amounts of water may be trapped with the hydrocarbons, and the closeness of the densities of water and of hydrocarbons makes gravitational separation inoperative. Moreover, the hydrocarbons tend to adhere to the walls of the containers used and/or to incorporate water.
Various long and tedious methods are presently used for allowing such an assay:                heating and evaporating water at atmospheric pressure or in vacuo, which does not solve the problem of determining the amount of water, which possibly remains trapped with the hydrocarbons;        assaying with X-rays, which requires very cumbersome apparatuses;        dilution of the hydrocarbons in a solvent such as toluene and then centrifugation for separating the water/oil emulsion: the quality of the water/oil separation is however very poor, and viewing the position of the interface is difficult.        
The article “No-solvent” Oil-in-Water Analysis, A Robust Alternative to Conventional Solvent Extraction Methods, of Brost et al., available on the Internet site of Turner Designs, describes a method for assaying crude in water, wherein a surfactant is added to the mixture in order to make a micro-emulsion of crude in water, and a fluorescence measurement is then carried out on this micro-emulsion. However, this method requires a large amount of surfactant and is limited to the measurement of small amounts of oil (less than 1%). The calibration curve is non-linear, and the surfactant itself generates fluorescence which has to be subtracted, whence a significant background noise.
Therefore there exists a real need for developing an improved and simplified method allowing the assay of hydrocarbons mixed with water, which may notably be applied reliably to heavy oils and without any limitation to a measurement of small amounts of hydrocarbons.